Binder For Secondary Battery Exhibiting Excellent Adhesion Force

ABSTRACT

Provided is a binder for secondary battery electrodes comprising polymer particles obtained by polymerizing three or more kinds of monomers wherein the polymer particles have a mean particle diameter of 0.3 mum to 0.7 mum. The binder exhibits superior adhesion force to electrode current collectors and excellent support force to the active material and basically improves safety of electrodes, thus providing a secondary battery with superior cycle characteristics.

TECHNICAL FIELD

The present invention relates to a binder for secondary batteryelectrodes. More specifically, the present invention relates to a binderfor secondary battery electrodes comprising polymer particles obtainedby polymerizing three or more kinds of monomers wherein the polymerparticles have a mean particle diameter of 0.3 μm to 0.7 μm.

BACKGROUND ART

Rapidly increasing use of fossil fuels has led to an increase in demandfor use of alternative or clean energy. In light of such trends,generation and storage of electricity using electrochemical reaction area very active area of research.

In recent years, representative examples of electrochemical devicesusing electrochemical energy are secondary batteries, and applicationrange thereof continues to expand.

Recently, technological development and increased demand associated withportable equipment such as portable computers, cellular phones andcameras have brought about an increase in the demand for secondarybatteries as energy sources. Among these secondary batteries, lithiumsecondary batteries having high energy density and operating electricpotential, long lifespan and low self-discharge have been activelyresearched and are commercially available and widely used.

In addition, increased interest in environmental issues has led to agreat deal of research into electric vehicles, hybrid electric vehiclesor the like as alternatives to vehicles using fossil fuels such asgasoline vehicles and diesel vehicles. These electric vehicles andhybrid electric vehicles generally use nickel-metal hydride secondarybatteries as power sources. However, a great deal of study associatedwith lithium secondary batteries with high energy density and dischargevoltage is currently underway and some are commercially available.

Conventional typical lithium secondary batteries use graphite as ananode active material. Lithium ions of a cathode are repeatedlyintercalated into and de-intercalated from the anode to realize chargeand discharge. The theoretical capacity of batteries may vary dependingupon the type of the electrode active material, but generally causedeterioration in charge and discharge capacity in the course of thecycle life of the battery.

The primary reason behind such phenomenon is that separation between anelectrode active material or separation between the electrode activematerial and a current collector due to volume variation of theelectrode, as batteries are charged and discharged, results ininsufficient realization of function of the active material. Inaddition, in the process of intercalation and de-intercalation, lithiumions intercalated into the anode cannot be sufficiently de-intercalatedand active sites of the anode are thus decreased. For this reason,charge/discharge capacity and lifespan of batteries may decrease as thebatteries are cycled.

In particular, in order to improve discharge capacity, in the case wherenatural graphite having a theoretical discharge capacity of 372 mAh/g isused in combination with a material such as silicon, tin or silicon-tinalloys having high discharge capacity, volume expansion of the materialconsiderably increases, in the course of charging and discharging, thuscausing isolation of the anode material from the electrode material. Asa result, battery capacity disadvantageously rapidly decreases overrepeated cycling.

Accordingly, there is an increasing demand in the art for binder andelectrode materials which can prevent separation between the electrodeactive material, or between the electrode active material and thecurrent collector upon fabrication of electrodes via strong adhesion andcan control volume expansion of electrode active materials upon repeatedcharging/discharging via strong physical properties, thus improvingstructural stability of electrodes and thus performance of batteries.

Polyvinylidene difluoride (PVdF), a conventional solvent-based binder,does not satisfy these requirements. Recently, a method for preparing abinder, in which styrene-butadiene rubber (SBR) is polymerized in anaqueous system to produce emulsion particles and the emulsion particlesare mixed with a neutralizing agent, or the like, is used and iscommercially available. Such a binder is advantageous in that it isenvironmentally friendly and reduces use of the binder and therebyincreasing battery capacity. However, this binder exhibits improvedadhesion maintenance due to the elasticity of rubber, but has no greateffect on adhesion force.

Accordingly, there is an increasing need for development of binderswhich improves cycle properties of batteries, contributes to structuralstability of electrodes and exhibits superior adhesion force.

DISCLOSURE Technical Problem

Therefore, the present invention has been made to solve the above andother technical problems that have yet to be resolved.

As a result of a variety of extensive and intensive studies andexperiments to solve the problems as described above, the inventors ofthe present invention developed a binder for secondary batteryelectrodes comprising polymer particles obtained by polymerizing threeor more kinds of monomers wherein the polymer particles have a meanparticle diameter of 0.3 μm to 0.7 μm, as described below, and confirmedthat the use of this binder contributes to improvement in cycleproperties of batteries due to superior adhesion force to electrodecurrent collectors and excellent support force to the active materialeven drying at high temperatures. The present invention was completedbased on this discovery.

Technical Solution

The binder for secondary battery electrodes according to the presentinvention comprises polymer particles obtained by polymerizing three ormore kinds of monomers wherein the polymer particles have a meanparticle diameter of 0.3 μm to 0.7 μm.

In a preferred embodiment, the three or more kinds of monomers are amixture of a (meth)acrylic acid ester monomer (a); at least one monomerselected from the group consisting of an acrylate monomer, a vinylmonomer and a nitrile monomer (b); and an unsaturated monocarbonic acidmonomer (c).

In this configuration, based on the total weight of the binder, the(meth)acrylic acid ester monomer (a) may be present in an amount of 15to 99% by weight, the monomer (b) may be present in an amount of 1 to60% by weight, and the unsaturated monocarbonic acid monomer (c) may bepresent in an amount of 1 to 25% by weight. More preferably, the monomer(a) is present in an amount of 25 to 90% by weight, the monomer (b) ispresent in an amount of 3 to 55% by weight, and the monomer (c) ispresent in an amount of 1 to 20% by weight. These content ranges may besuitably changed depending on the characteristics of the monomers andphysical properties of the binder.

For example, the (meth)acrylic acid ester monomer, as the monomer (a),may be at least one monomer selected from the group consisting of methylacrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, n-butylacrylate, isobutyl acrylate, n-amyl acrylate, isoamyl acrylate, n-ethylhexyl acrylate, 2-ethyl hexyl acrylate, 2-hydroxy ethyl acrylate, methylmethacrylate, ethyl methacrylate, propyl methacrylate, isopropylmethacrylate, n-butyl methacrylate, isobutyl methacrylate, n-amylmethacrylate, isoamyl methacrylate, n-hexyl methacrylate, n-ethyl hexylmethacrylate, 2-ethyl hexyl methacrylate, hydroxylethyl methacrylate andhydroxylpropyl methacrylate.

For example, the acrylate monomer, as the monomer (b), may be selectedfrom the group consisting of methacryloxy ethylethylene urea, β-carboxyethylacrylate, aliphatic monoacrylate, dipropylene diacrylate,ditrimethylolpropane tetraacrylate, hydroxyethyl acrylate,dipentaerythritol hexaacrylate, pentaerythritol triacrylate,pentaerythritol tetraacrylate, lauryl acrylate, cetyl acrylate, stearylacrylate, lauryl methacrylate, cetyl methacrylate and stearylmethacrylate.

The vinyl monomer, as the monomer (b), may be at least one selected fromthe group consisting of styrene, α-methylstyrene, β-methylstyrene,p-t-butylstyrene, divinyl benzene and mixtures thereof.

The nitrile monomer, as the monomer (b), may be at least one selectedfrom the group consisting of succinonitrile, sebaconitrile,fluoronitrile, chloronitrile, acrylonitrile, methacrylonitrile and thelike. More preferably, the nitrile monomer is at least one selected fromthe group consisting of acrylonitrile, methacrylonitrile and mixturesthereof.

The unsaturated monocarbonic acid monomer, as the monomer (c), may be atleast one selected from maleic acid, fumaric acid, methacrylic acid,acrylic acid, glutaconic acid, itaconic acid, tetrahydrophthalic acid,crotonic acid, nadic acid or a mixture thereof.

If desired, the binder may further comprise a (meth)acrylamide-basedmonomer. In this case, the (meth)acrylamide-based monomer may be presentin an amount of 0.1 to 10% by weight, based on the total weight of thebinder.

The binder of the present invention may further comprise a molecularweight controller and/or a cross-linking agent, as polymerizationadditives, in addition to the monomers.

The molecular weight controller may be selected from those well-known inthe art. Examples of the cross-linking agent include ethylene glycoldimethacrylate, diethylene glycol dimethacrylate, triethylene glycoldimethacrylate, 1,3-butanediol dimethacrylate, 1,6-hexanedioldimethacrylate, neopentyl glycol dimethacrylate, trimethylolpropanetrimethacrylate, trimethylol methane triacrylate, aryl methacrylate(AMA), triaryl isocyanurate (TAIC), triaryl amine (TAA), diaryl amine(DAA), polyethylene glycol dimethacrylate, polypropylene glycoldimethacrylate, polybutylene glycol diacrylate and the like.

The binder according to the present invention may be prepared byemulsion polymerization. The polymerization temperature andpolymerization time may be suitably determined depending on thepolymerization method or polymerization initiator employed, and forexample, the polymerization temperature may be from about 50° C. to 100°C. and the polymerization time may be from about 1 to about 20 hours.

Examples of the emulsifying agent used for emulsion polymerizationinclude oleic acid, stearic acid, lauric acid, fatty acid salts such assodium or potassium salts of mixed fatty acids, general anionicemulsifying agents such as rosin acid, and non-ionic emulsifying agentssuch as polyoxyethylene lauryl ether, polyoxyethylene glycol, andpolyoxyethylenenonylphenyl ether.

In order to fabricate electrodes of secondary batteries, a slurry iscoated and then dried. In a case of an aqueous binder, it is difficultto dry the slurry since water is evaporated. Also, it is important tocompletely dry the slurry, since moisture is a factor that inhibitsstability of batteries. As drying temperature increases, process timeshortens and drying speed increases, but a great amount of the binderalso moves to the coated surface. Accordingly, adhesive force isconsiderably decreased and it is thus difficult to increase a dryingtemperature.

On the other hand, the binder according to the present inventioncomprises polymer particles having a diameter larger than a meanparticle diameter of general polymer particles. Accordingly, arelatively great amount of polymer particles are present on a currentcollector after drying due to large particle diameter and an adhesiveforce to the current collector can be thus improved. However, anexcessively large particle diameter may have a negative effect onperformance of batteries such as conductivity. As defined above, theparticle diameter may be 0.3 μm to 0.7 μm, preferably 0.4 μm to 0.6 μm,more preferably 0.4 μm to 0.5 μm. When taking into consideration thefact that a polymer mean particle diameter is 0.1 μm to 0.3 μm ingeneral emulsion polymerization, the size of polymer particles in thebinder according to the present invention is large.

The mean particle diameter of the polymer can be for example controlledby various methods including changing the type or amount of emulsifyingagent or using polymerization through two steps, which can besufficiently implemented by those skilled in the emulsionpolymerization.

In the present invention, the binder may further comprise one or moreselected from the group consisting of a viscosity controller and afiller. The viscosity controller and the filler will be described indetail in the following.

The present invention provides a mix for secondary battery electrodescomprising the aforementioned binder and an electrode active materialcapable of intercalating and de-intercalating lithium.

The binder may be present in an amount of 0.5 to 20% by weight,preferably 1 to 10% by weight, based on the weight of the electrode mix.

The mix for secondary battery electrodes preferably further comprises aconductive material. The conductive material will be described in detailin the following.

The electrode active material is preferably a lithium transition metaloxide powder or a carbon powder.

Accordingly, the present invention provides an electrode for secondarybatteries in which the mix for secondary battery electrodes is appliedto a current collector.

The electrode may be fabricated by applying the mix for secondarybattery electrodes to a current collector, followed by drying androlling. The electrode for secondary batteries may be a cathode or ananode.

For example, the cathode is fabricated by applying a mixture consistingof a cathode active material, a conductive material and a binder to acathode current collector, followed by drying. The anode is fabricatedby applying a mixture consisting of an anode active material, aconductive material and a binder to an anode current collector, followedby drying. In some cases, the anode may comprise no conductive material.

The electrode active material is a material causing electrochemicalreaction in the electrode and is divided into a cathode active materialand an anode active material depending on the type of electrode.

The cathode active material is lithium transition metal oxide whichincludes two or more transition metals, and examples thereof include,but are not limited to, layered compounds such as lithium cobalt oxide(LiCoO₂) or lithium nickel oxide (LiNiO₂) substituted with one or moretransition metals; lithium manganese oxide substituted with one or moretransition metals; lithium nickel oxide represented by the formula ofLiNi_(1-y)M_(y)O₂ (in which M=Co, Mn, Al, Cu, Fe, Mg, B, Cr, Zn or Gaand includes one or more elements among the elements, 0.01≦y≦0.7);lithium nickel cobalt manganese composite oxides represented byLi_(1+z)Ni_(b)Mn_(c)Co_(1−(b+c+d))M_(d)O_((2-e))A_(e) such asLi_(1+z)Ni_(1/3)Co_(1/3)Mn_(1/3)O₂ or Li_(1+z)Ni_(0.4)Mn_(0.4)Co_(0.2)O₂(in which −0.5≦z≦0.5, 0.1≦b≦0.8, 0.1≦c≦0.8, 0≦d≦0.2, 0≦e≦0.2, b+c+d<1,M=Al, Mg, Cr, Ti, Si or Y, A=F, P or Cl); and olivine lithium metalphosphate represented by the formula Li_(1+x)M_(1−y)M′_(y)PO_(4-z)X_(z)(in which M=transition metal, preferably Fe, Mn, Co or Ni, M′=Al, Mg orTi, X═F, S or N, −0.5≦x≦0.5, 0≦y≦0.5, and 0≦z≦0.1).

Examples of the anode active material include carbon and graphitematerials such as natural graphite, artificial graphite, expandedgraphite, carbon fiber, incompletely-graphited carbon, carbon black,carbon nanotubes, perylene, activated carbon; metals alloyable withlithium, such as Al, Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Pd, Pt and Tiand compounds containing these elements; composites of carbon andgraphite materials with a metal and a compound thereof; andlithium-containing nitrides. Of these, a carbon-based active material, asilicon-based active material, a tin-based active material, or asilicon-carbon-based active material are more preferred. The materialmay be used alone or in combination of two or more thereof.

The conductive material serves to further improve conductivity of theelectrode active material and is commonly added in an amount of 0.01 to30% by weight, based on the total weight of the electrode mix. Anyconductive material may be used without particular limitation so long asit has suitable conductivity without causing adverse chemical changes inthe fabricated secondary battery. Examples of the conductive materialthat can be used in the present invention include conductive materials,including graphite such as natural or artificial graphite; carbon blackssuch as carbon black, acetylene black, Ketjen black, channel black,furnace black, lamp black and thermal black; carbon derivatives such ascarbon nanotubes or fullerene; conductive fibers such as carbon fibersand metallic fibers; metallic powders such as carbon fluoride powder,aluminum powder and nickel powder; conductive whiskers such as zincoxide and potassium titanate; conductive metal oxides such as titaniumoxide; and polyphenylene derivatives.

The current collector in the electrode is a material causingelectrochemical reaction and is divided into a cathode current collectorand an anode current collector depending on the type of electrode.

The cathode current collector is generally fabricated to have athickness of 3 to 500 μm. There is no particular limit to the cathodecurrent collector, so long as it has suitable conductivity withoutcausing adverse chemical changes in the fabricated battery. Examples ofthe cathode current collector include stainless steel, aluminum, nickel,titanium, sintered carbon, and aluminum or stainless steelsurface-treated with carbon, nickel, titanium, silver or the like.

The anode current collector is generally fabricated to have a thicknessof 3 to 500 μm. There is no particular limit to the anode currentcollector, so long as it has suitable conductivity without causingadverse chemical changes in the fabricated battery. Examples of theanode current collector include copper, stainless steel, aluminum,nickel, titanium, sintered carbon, and copper or stainless steelsurface-treated with carbon, nickel, titanium, silver, or the like, andaluminum-cadmium alloys.

These current collectors include fine irregularities on the surfacethereof so as to enhance adhesion to electrode active materials. Inaddition, the current collectors may be used in various forms includingfilms, sheets, foils, nets, porous structures, foams and non-wovenfabrics.

The mixture (electrode mix) of an electrode active material, aconductive material and a binder may further comprise at least oneselected from the group consisting of a viscosity controller and afiller.

The viscosity controller controls the viscosity of the electrode mix soas to facilitate mixing of the electrode mix and application thereof tothe current collector and may be added in an amount of 30% by weight orless, based on the total weight of the electrode mix. Examples of theviscosity controller include, but are not limited to,carboxymethylcellulose, polyacrylic acid and the like.

The filler is a component used to inhibit expansion of the electrode.There is no particular limit to the filler, so long as it does not causeadverse chemical changes in the fabricated battery and is a fibrousmaterial. Examples of the filler include olefin polymers such aspolyethylene and polypropylene; and fibrous materials such as glassfibers and carbon fibers.

The present invention also provides a lithium secondary batterycomprising the electrode.

The lithium secondary battery generally further comprises a separatorand a lithium salt-containing non-aqueous electrolyte, in addition tothe electrodes.

The separator is interposed between the cathode and the anode. As theseparator, an insulating thin film having high ion permeability andmechanical strength is used. The separator typically has a pore diameterof 0.01 to 10 μm and a thickness of 5 to 300 μm. As the separator,sheets or non-woven fabrics made of an olefin polymer such aspolypropylene and/or glass fibers or polyethylene, which have chemicalresistance and hydrophobicity, are used. When a solid electrolyte suchas a polymer is employed as the electrolyte, the solid electrolyte mayalso serve as both the separator and electrolyte.

The lithium salt-containing, non-aqueous electrolyte is composed of anon-aqueous electrolyte and a lithium salt.

Examples of the non-aqueous electrolytic solution that can be used inthe present invention include non-protic organic solvents such asN-methyl-2-pyrollidinone, propylene carbonate, ethylene carbonate,butylene carbonate, dimethyl carbonate, diethyl carbonate,gamma-butyrolactone, 1,2-dimethoxy ethane, tetrahydroxy franc, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide,dimethylformamide, dioxolane, acetonitrile, nitromethane, methylformate, methyl acetate, phosphoric acid triester, trimethoxy methane,dioxolane derivatives, sulfolane, methyl sulfolane,1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives,tetrahydrofuran derivatives, ether, methyl propionate and ethylpropionate.

The lithium salt is a material that is readily soluble in theabove-mentioned non-aqueous electrolyte and may include, for example,LiCl, LiBr, LiI, LiClO₄, LiBF₄, LiB₁₀Cl₁₀, LiPF₆, LiCF₃SO₃, LiCF₃CO₂,LiAsF₆, LiSbF₆, LiAlCl₄, CH₃SO₃Li, CF₃SO₃Li, (CF₃SO₂)₂NLi, chloroboranelithium, lower aliphatic carboxylic acid lithium, lithium tetraphenylborate and imides.

An organic solid electrolyte or an inorganic solid electrolyte may beused, if necessary.

Examples of the organic solid electrolyte utilized in the presentinvention, mention include polyethylene derivatives, polyethylene oxidederivatives, polypropylene oxide derivatives, phosphoric acid esterpolymers, poly agitation lysine, polyester sulfide, polyvinyl alcohols,polyvinylidene fluoride, and polymers containing ionic dissociationgroups.

Examples of the inorganic solid electrolyte utilized in the presentinvention, mention include nitrides, halides and sulfates of lithiumsuch as Li₃N, LiI, Li₅NI₂, Li₃N—LiI—LiOH, LiSiO₄, LiSiO₄—LiI—LiOH,Li₂SiS₃, Li₄SiO₄, Li₄SiO₄—LiI—LiOH and Li₃PO₄—Li₂S—SiS₂.

Additionally, in order to improve charge/discharge characteristics andflame retardancy, for example, pyridine, triethylphosphite,triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphorictriamide, nitrobenzene derivatives, sulfur, quinone imine dyes,N-substituted oxazolidinone, N,N-substituted imidazolidine, ethyleneglycol dialkyl ether, ammonium salts, pyrrole, 2-methoxy ethanol,aluminum trichloride or the like may be added to the non-aqueouselectrolyte. If necessary, in order to impart incombustibility, thenon-aqueous electrolyte may further comprise halogen-containing solventssuch as carbon tetrachloride and ethylene trifluoride. Further, in orderto improve high-temperature storage characteristics, the non-aqueouselectrolyte may further comprise carbon dioxide gas, fluoro-ethylenecarbonate (FEC), propene sultone (PRS) or fluoro-ethlene carbonate(FEC).

The secondary batteries according to the present invention may be usedfor battery cells used as power sources of small-sized devices and maybe used as unit batteries of middle- or large-sized modules comprising aplurality of battery cells used as power sources of middle- orlarge-sized devices.

Preferably, examples of the middle- or large-sized devices include powertools powered by battery-driven motors; electric vehicles includingelectric vehicles (EVs), hybrid electric vehicles (HEVs) and plug-inhybrid electric vehicles (PHEVs); electric two-wheeled vehiclesincluding electric bikes (E-bikes), electric scooters (E-scooters);electric golf carts, energy storage systems and the like.

Advantageous Effects

As apparent from the foregoing, the binder for secondary batteryelectrodes according to the present invention comprises polymerparticles obtained by polymerizing three or more kinds of monomerswherein the polymer particles have a mean particle diameter of 0.3 μm to0.7 μm, thus contributing to improvement in cycle properties ofbatteries due to superior adhesion force to electrode current collectorsand excellent support force to the active material even during drying athigh temperatures.

BEST MODE

Now, the present invention will be described in more detail withreference to the following Examples. These examples are provided only toillustrate the present invention and should not be construed as limitingthe scope and spirit of the present invention.

Example 1

Butyl acrylate (70 g), styrene (20 g) and acrylic acid (10 g) asmonomers and polyoxyethylene glycol and sodium lauryl sulfate asemulsifying agents were added to water containing potassium persulfateas a polymerization initiator, and these ingredients were mixed andpolymerized at 70° C. for about 10 hours. A binder for secondary batteryelectrodes containing polymer particles that are obtained bypolymerizing the monomers and have a mean particle diameter of 0.3 μmwas prepared through polymerization. The mean particle diameter of thepolymer can be controlled by controlling amounts of two emulsifyingagents. As an amount of the two emulsifying agents increases, particlediameter decreases and as an amount of the two emulsifying agentsdecreases, particle diameter increases.

Example 2

A binder for secondary battery electrodes comprising polymer particleshaving a mean particle diameter of 0.4 μm was prepared in the samemanner as Example 1, except that an amount of the emulsifying agent wasdecreased.

Example 3

A binder for secondary battery electrodes comprising polymer particleshaving a mean particle diameter of 0.5 μm was prepared in the samemanner as Example 1, except that an amount of the emulsifying agent wasdecreased.

Example 4

A binder for secondary battery electrodes polymer particles having amean particle diameter of 0.6 μm was prepared in the same manner asExample 1, except an amount of the emulsifying agent was decreased.

Example 5

A binder for secondary battery electrodes comprising polymer particleshaving a mean particle diameter of 0.3 μm was prepared in the samemanner as Example 1, except that acrylamide (1 g) was further used as amonomer.

Example 6

A binder for secondary battery electrodes comprising polymer particleshaving a mean particle diameter of 0.3 μm was prepared in the samemanner as Example 1, except that acrylonitrile was used as the monomerinstead of styrene.

Comparative Example 1

A binder for secondary battery electrodes comprising polymer particleshaving a mean particle diameter of 0.1 μm was prepared in the samemanner as Example 1, except that an amount of the emulsifying agent wasincreased.

Comparative Example 2

A binder for secondary battery electrodes comprising polymer particleshaving a mean particle diameter of 0.2 μm was prepared in the samemanner as Example 1, except that an amount of the emulsifying agent wasincreased.

Comparative Example 3

A binder for secondary battery electrodes comprising polymer particleshaving a mean particle diameter of 0.2 μm was prepared in the samemanner as Example 6, except that an amount of the emulsifying agent wasincreased.

Comparative Example 4

A binder for secondary battery electrodes comprising polymer particleshaving a mean particle diameter of 0.8 μm was prepared in the samemanner as Example 1, except that an amount of the emulsifying agent wasconsiderably decreased.

Experimental Example 1 Adhesion Force Test

First, for the binders of Examples 1 to 6 and the binders of ComparativeExamples 1 to 4, an anode active material, a conductive material, athickener, and a binder were mixed in a ratio of 95:1:1:3 to prepare aslurry and the slurry was coated on an Al foil to fabricate anelectrode. The electrode was dried at 90° C. and 120° C.

The electrode thus fabricated was pressed to a predetermined thickness,cut into a predetermined size and fixed on a glass slide and 180 degreepeel strength was measured, while the current collector was peeled off.The results thus obtained are shown in Table 1. Evaluation was based onan average of five or more peel strengths.

TABLE 1 Adhesion force (gf/cm) Drying at 90° C. Drying at 120° C. Ex. 128 25 Ex. 2 32 27 Ex. 3 34 29 Ex. 4 30 25 Ex. 5 30 27 Ex. 6 31 26 Comp.Ex. 1 19 15 Comp. Ex. 2 23 17 Comp. Ex. 3 20 17 Comp. Ex. 4 21 20

As can be seen from Table 1 above, electrodes employing the binders ofExamples 1 to 6 according to the present invention exhibitedconsiderably high adhesion force, as compared to electrodes employingthe binders of Comparative Examples 1 to 4. In particular, it can beseen from comparison between the Comparative Example 2 and Example 1,and between Comparative Example 3 and Example 6, that when the meanparticle diameter is 0.3 μm or more, an adhesion force is considerablyincreased. Also, variation in adhesion force according to particlediameter is great at 120° C., as compared to at 90° C. The reason forthis is that when the mean particle diameter is higher than 0.3 μm,movement of the binder is considerably decreased during drying than whenthe mean particle diameter is lower than 0.3 μm, and the effectincreases, as temperature increases. The reason is thought that adecrease in adhesion force caused by movement of the binder is greaterthan an increase in adhesion force caused by specific surface area ofthe binder.

On the other hand, in Comparative Example 4 in which although the meanparticle diameter is large, an adhesion force is low, when the meanparticle diameter of the binder is 0.8 μm, movement of the binder isdecreased, but a specific surface area of the binder is extremelydecreased, an area where the binder contacts an active material isdecreased and the adhesion force is thus greatly decreased.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A binder for secondary battery electrodes comprising polymerparticles obtained by polymerizing three or more kinds of monomerswherein the polymer particles have a mean particle diameter of 0.3 μm to0.7 μm.
 2. The binder according to claim 1, wherein the polymerparticles have a mean particle diameter of 0.4 μm to 0.6 μm.
 3. Thebinder according to claim 1, wherein the polymer particles have a meanparticle diameter of 0.4 μm to 0.5 μm.
 4. The binder according to claim1, wherein the three or more kinds of monomers are a mixture of a(meth)acrylic acid ester monomer (a); at least one monomer selected fromthe group consisting of an acrylate monomer, a vinyl monomer and anitrile monomer (b); and a unsaturated monocarbonic acid monomer (c). 5.The binder according to claim 4, wherein the (meth)acrylic acid estermonomer (a) is present in an amount of 15 to 99% by weight, the monomer(b) is present in an amount of 1 to 60% by weight, and the unsaturatedmonocarbonic acid monomer (c) is present in an amount of 1 to 25% byweight, based on the total weight of the binder.
 6. The binder accordingto claim 4, wherein the (meth)acrylic acid ester monomer is at least onemonomer selected from the group consisting of methyl acrylate, ethylacrylate, propyl acrylate, isopropyl acrylate, n-butyl acrylate,isobutyl acrylate, n-amyl acrylate, isoamyl acrylate, n-ethyl hexylacrylate, 2-ethyl hexyl acrylate, 2-hydroxy ethyl acrylate, methylmethacrylate, ethyl methacrylate, propyl methacrylate, isopropylmethacrylate, n-butyl methacrylate, isobutyl methacrylate, n-amylmethacrylate, isoamyl methacrylate, n-hexyl methacrylate, n-ethyl hexylmethacrylate, 2-ethyl hexyl methacrylate, hydroxyethyl methacrylate andhydroxypropyl methacrylate.
 7. The binder according to claim 4, whereinthe acrylate monomer is selected from the group consisting ofmethacryloxy ethylethylene urea, β-carboxy ethylacrylate, aliphaticmonoacrylate, dipropylene diacrylate, ditrimethylolpropanetetraacrylate, hydroxyethyl acrylate, dipentaerythritol hexaacrylate,pentaerythritol triacrylate, pentaerythritol tetraacrylate, laurylacrylate, cetyl acrylate, stearyl acrylate, lauryl methacrylate, cetylmethacrylate and stearyl methacrylate.
 8. The binder according to claim4, wherein the vinyl monomer is at least one selected from the groupconsisting of styrene, α-methylstyrene, β-methylstyrene,p-t-butylstyrene, and divinyl benzene.
 9. The binder according to claim4, wherein the nitrile monomer is at least one selected from the groupconsisting of succinonitrile, sebaconitrile, fluoronitrile,chloronitrile, acrylonitrile and methacrylonitrile.
 10. The binderaccording to claim 4, wherein the unsaturated monocarbonic acid monomeris at least one selected from maleic acid, fumaric acid, methacrylicacid, acrylic acid, glutaconic acid, itaconic acid, tetrahydrophthalicacid, crotonic acid, isocrotonic acid and nadic acid.
 11. The binderaccording to claim 1, wherein the binder further comprises one or moreselected from the group consisting of a viscosity controller and afiller.
 12. A mix for secondary battery electrodes comprising: thebinder for secondary battery electrodes according to claim 1; and anelectrode active material capable of intercalating and de-intercalatinglithium.
 13. The mix according to claim 12, wherein the electrode activematerial is a lithium transition metal oxide powder or a carbon powder.14. An electrode for secondary batteries, in which the mix forelectrodes according to claim 12 is applied to a current collector. 15.The electrode according to claim 14, wherein the current collector has athickness of 3 to 500 μm and includes fine irregularities on the surfacethereof.
 16. A lithium secondary battery comprising the electrode forsecondary batteries according to claim 15.